Pressure-treated lumber for a deck has to match its exposure. Above-ground wood (AWPA Use Category UC3B) is fine for decking and railings. Posts, and framing that sits near the soil, stays damp, or would be hard to replace, should be ground-contact wood (UC4A). Buy No. 2 grade or better, and fasten it only with hot-dip galvanized or stainless steel hardware, because modern copper-based treatments corrode ordinary steel.
The use category is printed on the tag stapled to the end of each board. This guide explains how to read that tag, where each rating belongs on a deck, and the fastener rules that go with treated wood.
What “pressure-treated” means
Treated lumber is softwood, usually southern yellow pine in the eastern US and species such as hem-fir or Douglas fir in the West. It is loaded with preservative under pressure in a sealed cylinder, which drives the chemical deep into the wood to resist decay fungi and insects.
Most residential lumber today uses copper-based preservatives. The USDA Forest Products Laboratory lists the common ones as alkaline copper quat (ACQ), copper azole (CA-B) and micronized copper azole (MCA). Some products use copper-free formulas instead. You do not have to pick a chemical. What matters on a deck is the use category and retention on the tag, because those tell you what exposure the wood was treated for.
Reading the end tag
The American Wood Protection Association (AWPA) publishes the standards that treaters follow. Its residential infographic breaks down a typical end tag. It shows:
- Brand or company name and the treating plant’s location
- Standard and use category, for example “AWPA U1, UC3B”
- Preservative, for example “CA-C”
- Retention, the amount of preservative per cubic foot of wood (pcf). The AWPA example shows “.06 pcf”
- Exposure, such as “ABOVE GROUND” or “GROUND CONTACT”
- Inspection agency logo, showing third-party quality control under the American Lumber Standard Committee (ALSC)
Retention values differ by preservative, so don’t compare numbers across chemicals. Compare the use category instead. For a given preservative, wood treated for ground contact generally carries a higher retention than above-ground wood.
Separate from the treatment tag, each board should carry a grade stamp from a grading agency. Anne Arundel County’s IRC-based deck guide, like many local guides, requires deck lumber to be No. 2 grade or better, preservative-treated per IRC R317, with ground-contact wood labeled for that use.
UC3B vs. UC4A: which goes where
The AWPA use categories that matter for decks:
| Category | AWPA description | Typical deck use |
|---|---|---|
| UC3B | Above ground, exterior, uncoated or poor water runoff | Decking, railings, balusters, joists and beams with good clearance and airflow |
| UC4A | Ground contact, general use | Posts, framing near soil or in damp spots, joists and beams that are critical and hard to replace |
| UC4B | Ground contact, heavy duty | Permanent wood foundations, severe sites, critical structural poles |
Sources: AWPA residential infographic; AWPA U1 standard excerpt.
The AWPA infographic lists deck boards, railings, stairs, and joists and beams under Use Category 3B or higher. It lists posts and ledgers under 4A or higher. It adds a footnote that above-ground components “may be required to be preserved for ground contact,” including joists and beams that are difficult to replace and critical to the structure.
When above-ground wood isn’t enough
The AWPA U1 standard spells out conditions where sawn lumber that is physically above the ground must still be treated to UC4A. Paraphrasing its Note 1 (AWPA U1 excerpt):
- Soil, vegetation, leaf litter or other debris is likely to build up against the wood
- The construction or plantings will not let air circulate underneath the deck and between boards
- The wood is installed less than 6” above final grade and supported on permeable materials without a moisture barrier
- The wood touches untreated, non-durable wood or older construction showing decay
- The wood is wetted frequently, as by a fixed sprinkler or on a floating dock
- The project is in a tropical climate
A separate footnote in U1 says joists and beams “shall be treated to requirements for UC4A when they are difficult to maintain, repair or replace and are critical to the performance and safety of the entire system.”
On a real deck, that covers a lot of framing. A low deck with joists a few inches above soil, or a deck with skirting and planters around it, has framing that meets several of these conditions. Joists under a deck are also hard to replace once the decking is on. Many builders now simply buy ground-contact lumber for all framing and reserve UC3B for the decking and rails. The extra cost is small compared with re-framing a deck.
Field cuts
Treatment does not always reach the center of a board, especially in larger sizes. Every cut, notch and drilled hole exposes wood that may be untreated. The American Wood Council’s DCA 6 guide says all cuts shall be field treated with an approved preservative, such as copper naphthenate. This is especially important on post bottoms and joist ends. Brush it on generously and let it soak in.
Grades and choosing boards
Lumber grade describes strength and appearance: knot size, wane, splits and slope of grain. The joist and beam span tables in the IRC and DCA 6 assume No. 2 grade, so that is the practical minimum for framing. Using a lower grade means the published spans no longer apply.
For decking and railings, appearance matters more. Lumberyards often sell premium or select grades with fewer knots, and dedicated decking profiles such as 5/4 radius-edge boards.
When you pick through a stack:
- Sight down each board. Reject boards with strong crook (edge bow) for joists, or crown them all the same direction (crown up).
- Avoid large loose knots near the edges of joists and in the middle of deck boards.
- Check the moisture. Most treated lumber is sold wet and shrinks as it dries, mostly in width and thickness. Kiln-dried-after-treatment (KDAT) lumber costs more but moves less after installation. Wet boards butted tight will open a gap as they dry, which affects how you space decking. See the deck board spacing guide.
Fasteners and connectors
Copper-based preservatives corrode plain steel and thin zinc coatings much faster than older treatments did. The IRC addresses this directly. Section R317.3.1 requires that fasteners for preservative-treated wood, including nuts and washers, be “hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper”. For connectors, it defers to the manufacturer’s recommendations or a minimum of G185 galvanizing.
The practical rules, drawn from DCA 6 and Simpson Strong-Tie’s treated-wood FAQ:
- Screws, nails, bolts: hot-dip galvanized (ASTM A153) or stainless steel. Fasteners other than nails may instead be mechanically galvanized to ASTM B695 Class 55. Simpson notes that electroplated and mechanically galvanized coatings “should not be considered to be hot-dip galvanized.”
- Joist hangers and connectors: G185 hot-dip galvanized (Simpson sells this as ZMAX) or stainless steel. Standard G90 hangers are not meant for treated lumber.
- Coastal sites: DCA 6 requires stainless steel grade 304 or 316 for fasteners and connectors exposed to salt water or within 300 ft of a saltwater shoreline. Simpson notes that Type 316 holds up better in high-chloride environments.
- Don’t mix metals. DCA 6 says fasteners and connectors must be of the same corrosion-resistant material. Simpson states plainly that “you should not use galvanized fasteners with stainless steel connectors.”
- Watch aluminum. DCA 6 warns that aluminum flashing should not touch lumber treated with copper preservatives such as ACQ or copper azole. Use vinyl or compatible flashing at the ledger instead.
Deck screws sold for treated lumber are usually coated and labeled for use with ACQ or other copper treatments. Read the box. The deck screw calculator estimates how many you need for the decking.
Ordering treated lumber
A sensible order for a typical deck:
- Posts: ground contact (UC4A), usually 6x6. DCA 6 uses 6x6 as its minimum post size.
- Beams, joists, rim and ledger: ground contact if any of the AWPA conditions apply, or simply to be safe. The deck framing calculator counts joists, rims and blocking, and the joist spacing guide explains span choices.
- Decking, stair treads, rails and balusters: above ground (UC3B) is acceptable.
- Hardware: G185 or stainless hangers and post bases, plus matching fasteners.
The lumber calculator turns a cut list into board counts by length. Add a few spare boards for rejects and cutting errors. Then check your plan with your local building department, since many jurisdictions publish their own deck requirements. This site provides general information, not engineering advice.
Frequently asked questions
Can I use above-ground treated lumber for deck joists?
AWPA lists joists and beams under UC3B in normal conditions. But its U1 standard requires UC4A when framing is within 6” of grade, poorly ventilated, likely to collect debris, or hard to replace and critical to the structure. Many deck joists meet at least one of those conditions, so ground-contact lumber is the safer choice for framing.
Do I need to treat the cut ends of pressure-treated lumber?
Yes. Preservative may not reach the core of the board, so cuts and holes expose less-protected wood. DCA 6 requires field treatment of cuts with an approved preservative such as copper naphthenate. Pay the most attention to post bottoms and any end that will stay damp.
Are stainless steel screws required for treated wood?
Not everywhere. The IRC allows hot-dip galvanized, stainless steel, silicon bronze or copper fasteners. Stainless is the most corrosion-resistant choice, and DCA 6 requires it within 300 ft of salt water. Inland, many builders use hot-dip galvanized or coated screws rated for treated lumber.
Why is my new treated lumber so wet and heavy?
Most treated lumber is shipped soon after treatment, while it still holds water from the process. It shrinks as it dries over the following weeks or months. Kiln-dried-after-treatment lumber is drier at purchase. Let wet wood dry before staining. The deck staining guide explains how to tell when it is ready.